The background description provided here is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Fabrication of substrates such as semiconductor wafers typically requires multiple processing steps that may include material deposition, planarization, feature patterning, feature etching, and/or feature cleaning. These processing steps are typically repeated one or more times during processing of the substrate.
As semiconductor devices continue to scale down to smaller feature sizes, high aspect ratio (HAR) structures are increasingly required to achieve desired device performance objectives. The use of the HAR structures creates challenges for some of the substrate processing steps. For example, wet processes such as etching and cleaning pose problems for the HAR structures due to capillary forces that are generated during drying of the substrate. The strength of the capillary forces depends upon surface tension, a contact angle of the etching, cleaning, or rinsing fluids that are being dried, feature spacing and/or an aspect ratio of the structures. If the capillary forces generated during drying are too high, the HAR structures will become strained or collapse onto each other and stiction may occur, which severely degrades device yield.
To solve this problem, one approach uses rinsing liquids that have a lower surface tension than deionized water to prevent the structures from collapsing. While generally successful for relatively low aspect ratio structures, this approach has the same collapse and stiction issues as methods that use deionized water. The rinsing fluids still possess a finite amount of surface tension that generates forces during drying that are still too strong for the fragile HAR structures.
An alternative approach for drying HAR structures involves dissolving and flushing the rinsing fluid with a supercritical fluid. Supercritical fluids are free of surface tension when processed correctly. However, several technical and manufacturing challenges arise when using the supercritical fluids. The challenges include high equipment and safety costs, long process times, variable solvent quality during the process, extreme sensitivity due to the diffuse and tunable nature of the fluid, and wafer defectivity/contamination issues arising from the interaction of the supercritical fluid with components of the processing chamber.
Another strategy for preventing collapse of high aspect ratio structures is to add a permanent mechanical bracing structure that supports the structures. There are several tradeoffs with this approach including higher cost and process complexity that negatively impact throughput and yield. Furthermore, permanent mechanical bracing structures are limited to supporting certain types of HAR structures.
Freeze drying has also been proposed as an alternative approach for drying HAR structures. Freeze drying eliminates collapse by initially freezing the solvent and then directly sublimating under vacuum. Freeze drying avoids the liquid/vapor interface, which minimizes capillary forces. While showing promise, freeze drying has relatively high cost, low throughput and high defects as compared to competing approaches.
Surface modification of sidewalls of the HAR structures may also be performed. In this approach, small molecules may be chemically bonded to the sidewalls of the HAR structures. The small molecules improve collapse performance by either preventing the stiction of materials when they make contact or by altering a contact angle of the wet chemistry to minimize Laplace pressure. Surface modification does not fully eliminate the drying forces and the structures deform during the drying process, which may cause damage. Furthermore, when surface materials are changed, new tailored molecules are required to bond to the sidewalls of the HAR structures.